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| United States Patent |
6,227,629
|
|
Yoshida
, et al.
|
May 8, 2001
|
Brake force control apparatus
Abstract
A brake force control apparatus is provided which generates a brake force
larger than that of a normal time when an emergency braking is required,
and realizes an operational feel giving no incongruous feel. It is
determined whether or not an emergency braking operation was performed in
accordance with a master cylinder pressure PM/C and a rate of change
.DELTA.PM/C thereof. A plurality of start conditions (100, 112, 118) are
set by assuming various conditions. A brake assist control is started
(114) when a start condition selected in accordance with a state of motion
of a vehicle is satisfied.
| Inventors:
|
Yoshida; Hiroaki (Mishima, JP);
Hashimoto; Yoshiyuki (Susono, JP)
|
| Assignee:
|
Toyota Jidosha Kabushiki Kaisha (Toyota, JP)
|
| Appl. No.:
|
155769 |
| Filed:
|
December 17, 1998 |
| PCT Filed:
|
April 4, 1997
|
| PCT NO:
|
PCT/JP97/01165
|
| 371 Date:
|
December 17, 1998
|
| 102(e) Date:
|
December 17, 1998
|
| PCT PUB.NO.:
|
WO97/37880 |
| PCT PUB. Date:
|
October 16, 1997 |
Foreign Application Priority Data
| Current U.S. Class: |
303/155; 303/113.4 |
| Intern'l Class: |
B60T 007/12 |
| Field of Search: |
303/113.4,155,3,15,20
|
References Cited
U.S. Patent Documents
| 4757449 | Jul., 1988 | Kurihara et al. | 303/155.
|
| 5158343 | Oct., 1992 | Reichelt et al.
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| 5261730 | Nov., 1993 | Steiner et al.
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| 5350225 | Sep., 1994 | Steiner et al.
| |
| 5367942 | Nov., 1994 | Nell et al.
| |
| 5427442 | Jun., 1995 | Heibel.
| |
| 5445444 | Aug., 1995 | Rump et al. | 303/155.
|
| 5492397 | Feb., 1996 | Steiner et al.
| |
| 5496099 | Mar., 1996 | Resch.
| |
| 5499866 | Mar., 1996 | Brugger et al.
| |
| 5513906 | May., 1996 | Steiner | 303/155.
|
| 5535123 | Jul., 1996 | Rump et al.
| |
| 5549369 | Aug., 1996 | Rump et al. | 303/155.
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| 5556173 | Sep., 1996 | Steiner et al.
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| 5564797 | Oct., 1996 | Steiner et al.
| |
| 5567021 | Oct., 1996 | Gaillard.
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| 5584542 | Dec., 1996 | Klarer et al.
| |
| 5586814 | Dec., 1996 | Steiner.
| |
| 5658055 | Aug., 1997 | Dieringer et al.
| |
| 5660448 | Aug., 1997 | Kiesewetter et al. | 303/155.
|
| 5669676 | Sep., 1997 | Rump et al. | 303/155.
|
| 5719769 | Feb., 1998 | Brugger et al.
| |
| 5720532 | Feb., 1998 | Steiner et al. | 303/155.
|
| 5772290 | Jun., 1998 | Heibel et al.
| |
| 5779329 | Jul., 1998 | Takeshima | 303/155.
|
| 5833327 | Nov., 1998 | Kozakai | 303/113.
|
| Foreign Patent Documents |
| WO96/6763 | Mar., 1996 | EP.
| |
| 0711695 | May., 1996 | EP.
| |
| 2282649 | Dec., 1995 | GB.
| |
| 2295209 | May., 1996 | GB.
| |
| 61-268560 | Nov., 1986 | JP.
| |
| 3-227766 | Oct., 1991 | JP.
| |
| 4-121260 | Apr., 1992 | JP.
| |
| 5-97022 | Apr., 1993 | JP.
| |
| 07 76267 | Mar., 1995 | JP.
| |
| 07 165038 | Jun., 1995 | JP.
| |
| 07 156786 | Jun., 1995 | JP.
| |
| 7-329766 | Dec., 1995 | JP.
| |
| 8-34326 | Feb., 1996 | JP.
| |
| 8-40229 | Feb., 1996 | JP.
| |
| 8-295224 | Nov., 1996 | JP.
| |
Primary Examiner: Graham; Matthew C.
Attorney, Agent or Firm: Kenyon & Kenyon
Claims
What is claimed is:
1. A brake force control apparatus controlling a braking system to generate
a brake force, the brake force control apparatus comprising:
operational speed detecting means for detecting an operational speed of a
brake pedal;
operational amount detecting means for detecting an operational amount
parameter associated with an amount of travel of the brake pedal; and
brake force generating means for generating a normal brake force component
based on the operational amount parameter and, when the operational speed
is at least as great as a first threshold speed and the operational amount
parameter is at least as great as a first operation threshold value, the
brake force generating means generates an assist brake force component to
be applied in addition to the normal brake force component.
2. The brake force control apparatus as claimed in claim 1, wherein the
operational amount parameter includes an amount of pedal stroke and the
operational amount parameter threshold value includes a pedal stroke
threshold value.
3. The brake force control apparatus as claimed in claim 1, wherein the
operational amount parameter includes a master cylinder pressure and the
operational amount parameter threshold value includes a master cylinder
pressure threshold value.
4. The brake force control apparatus as claimed in claim 1, wherein the
operational amount parameter includes a vehicle deceleration and the
operational amount parameter threshold value includes a vehicle
deceleration threshold value.
5. The brake force control apparatus as claimed in claim 1, wherein the
operational amount parameter includes a brake pedal pressing force and the
operational amount parameter threshold value includes a brake pedal
pressing force threshold value.
6. A brake force control apparatus controlling a braking system to generate
a brake force, the brake force control apparatus comprising:
operational speed detecting means for detecting an operational speed of a
brake pedal; and
brake force generating means for generating a normal brake force component
based on an operational amount parameter associated with an amount of
travel of a brake pedal and, when the operational speed is at least as
great as a first threshold speed and no greater than a second threshold
speed, the brake force generating means generates an assist brake force
component to be applied in addition to the normal brake force component,
wherein the second threshold speed is greater than the first threshold
speed.
7. The brake force control apparatus as claimed in claim 6, further
comprising start condition changing means for changing the first threshold
speed based on an amount of time elapsed from a time at which the brake
pedal is pressed.
8. The brake force control apparatus as claimed in claim 7, wherein the
start condition changing means decreases the first threshold speed.
9. The brake force control apparatus as claimed in claim 6, wherein the
brake force generating means generates the assist brake force only when
the operational amount parameter is at least as great as a first threshold
value.
10. The brake force control apparatus as claimed in claim 9, further
comprising start condition changing means for changing the first threshold
operation value in response to an amount of time elapsed after the brake
pedal is pressed.
11. The brake force control apparatus as claimed in claim 10, wherein the
start condition changing means increases the first threshold operation
value.
12. The brake force control apparatus as claimed in claim 9, further
comprising start condition changing means for changing the first threshold
operation value based on a vehicle speed.
13. The brake force control apparatus as claimed in claim 12, wherein the
start condition changing means changes decreases the first threshold
operation value.
14. The brake force control apparatus as claimed in claim 6, further
comprising start condition changing means for changing the second
threshold speed based on an amount of time elapsed after the brake pedal
is pressed.
15. The brake force control apparatus as claimed in claim 14, wherein the
start condition changing means decreases the second threshold speed.
16. The brake force control apparatus as claimed in claim 6, further
comprising BA start prohibiting means for prohibiting the determination of
the assist brake force when a vehicle speed is smaller than a
predetermined value.
17. The brake force control apparatus as claimed in claim 6, further
comprising first assist brake force canceling means for canceling
application of the assist brake force to the braking system when an amount
of operation of the brake pedal exceeds a predetermined value after the
assist brake force is determined.
18. The brake force control apparatus as claimed in claim 6, further
comprising a second assist brake force canceling means for canceling
application of the assist brake force to the braking system when the
operational speed exceeds a predetermined value after the assist brake
force is determined.
Description
TECHNICAL FIELD
The present invention relates to a brake force control apparatus and, more
particularly, to a brake force control apparatus which generates, when an
emergency braking is required, a brake force greater than that generated
in an ordinary time.
BACKGROUND ART
Conventionally, for example, as disclosed in Japanese Laid-Open Patent
Application 4-121260, a brake force control apparatus which generates,
when an emergency braking is required, a brake force greater than that
generated in a normal time is known. The above-mentioned conventional
apparatus comprises a control circuit which generates a drive signal
corresponding to an operational speed of a brake pedal and a fluid
pressure generating mechanism which generates a brake fluid pressure
corresponding to the drive signal generated by the control circuit.
The control circuit determines that, when an operational speed of a brake
pedal is less than a predetermined value, the brake pedal is not normally
operated. In this case, the fluid pressure generating mechanism is
controlled so that a brake fluid pressure corresponding to a brake
pressing force is generated. Hereinafter, this control is referred to as a
normal control. Additionally, the control circuit determines that, when an
operational force of the brake pedal exceeds a predetermined value, an
emergency braking is required by the driver. In this case, the fluid
pressure generating mechanism is controlled so that a brake fluid pressure
is maximized. Hereinafter, this control is referred to as a brake assist
control. Thus, according to the above-mentioned conventional apparatus, a
brake force corresponding to a brake pressing force can be generated in a
normal time, and a large brake force can be immediately generated in an
emergency.
In the above-mentioned conventional apparatus, a normal braking operation
and an operation requiring an emergency braking are discriminated in
accordance with an operational speed of the brake pedal. Generally, the
operational speed of the brake pedal when an emergency braking is required
is higher than that of the normal braking operation. Thus, according to
the above-mentioned discriminating method, the operation requiring an
emergency braking and the operation requiring a normal brake can be
discriminated with high accuracy.
However, for the purpose of obtaining a suitable deceleration, depending on
travel circumstances, the brake pedal may be slightly pressed at a high
speed without an intention to rapidly decelerate the vehicle.
(Hereinafter, this operation is referred to as a small high-speed
operation). In an apparatus in which the emergency braking and the normal
brake are discriminated based on only an operational speed of the brake
pedal such as in the above-mentioned apparatus, when the above-mentioned
small high-speed operation is performed, it is possible that an erroneous
determination is made that an emergency braking is required.
Additionally, in the above-mentioned apparatus, when the brake pedal is
pressed at an operational speed exceeding a predetermined value, the fluid
pressure generating mechanism is switched from a state for realizing the
normal control to a state for realizing the brake assist control. Such a
switching operation requires a certain time delay. Accordingly, when a
brake fluid pressure at a high-pressure level can be obtained by
continuing the normal control when a driver is highly skilled, it is
preferred that the switching to the brake assist control not be performed.
However, in the above-mentioned conventional apparatus, when an operational
speed of the brake pedal exceeds a predetermined speed, the switching to
the brake assist control is always performed. In this regard, the
above-mentioned conventional apparatus may give an unpleasant feel to the
driver due to that control when the driver's skill level is high.
Additionally, depending on travel circumstances of the vehicle, there may
be a case in which a braking operation is started gently and, thereafter,
the brake pedal is pressed at a high speed, due to an emergency braking
being required. (Hereinafter, such an operation is referred to as a spurt
operation.) When the above-mentioned spurt operation is performed, a brake
fluid pressure has already been increased to a certain level at a stage in
which the brake pedal is pressed at a high speed. Accordingly, the
operational speed of the brake pedal in the spurt operation is not as high
as the operational speed of the brake pedal in an ordinary emergency
braking.
However, in the above-mentioned conventional apparatus, it is always
determined whether the braking operation by the driver is a normal braking
operation or an operation requiring an emergency braking based on the
determination as to whether or not the operational speed of the brake
pedal exceeds the constant threshold value. Accordingly, the
above-mentioned conventional apparatus has a characteristic in which the
switching from the normal control to the brake assist control tends not to
be performed when the brake pedal is subjected to the spurt operation.
As mentioned above, the above-mentioned conventional apparatus may
generated a difference between a driver's intention and contents of the
control to be performed, since the switching between the normal control
and the brake assist control is performed based on the determination as to
whether or not the operational speed of the brake pedal exceeds the
constant threshold value.
DISCLOSURE OF INVENTION
The present invention is invented in view of the above-mentioned point, and
it is an object of the present invention to provided a brake force control
apparatus which generates an appropriate brake force conforming to the
driver's intention without an incongruous feel in practice under a
condition in which the normal brake is required and a condition in which
an emergency braking is required.
A brake force control apparatus which achieves the above-mentioned object
selectively performs the normal control for generating a brake force
corresponding to a brake pressing force and the brake assist control for
generating a brake force greater than that of the normal control.
Additionally, the above-mentioned brake force control apparatus comprises
an operational speed detecting mechanism for detecting an amount of
operation of a brake pedal and a control start time determining mechanism
for determining a start time of the brake assist control based on an
operational speed and the amount of operation of the brake pedal.
In the present invention, the start time of the brake assist control is
determined based on the operational speed and the amount of operation of
the brake pedal. When the driver requires an emergency braking, the brake
pedal is operated at a high speed with a large travel. Accordingly, by
rendering both the operational speed and the amount of operation of the
brake pedal as parameters, the driver's intention can be detected with
good accuracy. Thus, according to the brake force control apparatus of the
present invention, the brake assist control can be appropriately performed
when the driver is actually requiring an emergency braking.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a system structure diagram of a brake force control apparatus
according to an embodiment of the present invention;
FIG. 2 is an illustration for showing a change in a brake pressing force
achieved under various circumstances;
FIG. 3 is an illustration for showing a start condition used for
determining whether brake assist control is started in the brake force
control apparatus shown in FIG. 1;
FIG. 4 is a flowchart of an example of a control routine performed in the
brake force control apparatus shown in FIG. 1;
FIG. 5 is a flowchart of an example of another control routine performed in
the brake force control apparatus shown in FIG. 1;
FIG. 6, is a system structure diagram of a brake force control apparatus
according to a second embodiment of the present invention;
FIG. 7 is an illustration for showing a vacuum booster used in the brake
force control apparatus shown in FIG. 6 and a peripheral structure
thereof; and
FIG. 8 is a system structure of a brake force control apparatus according
to a third embodiment of the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 is a system structure diagram of a brake force control apparatus
according to an embodiment of the present invention. The brake force
control apparatus shown in FIG. 1 is controlled by an electronic control
unit 10 (hereinafter, referred to as ECU 10). The brake force control
apparatus comprises a pump 12. The pump 12 has a motor 14 as a power
source thereof. An inlet port 12a of the pump 12a communicates with a
reservoir tank 16. An accumulator 20 communicates with an outlet port 12b
of the pump via a check valve 18. The pump 12 delivers brake fluid in the
reservoir tank 16 from the outlet port 12b so that a predetermined
pressure is always accumulated in the accumulator 20.
The accumulator 20 communicates with a high-pressure port 24a of a
regulator 24 via a high-pressure passage 22, and communicates with a
regulator switching solenoid 26 (hereinafter, referred to as STR 26). The
regulator 24 has a low-pressure port 24b and a control fluid pressure port
24c. The low-pressure port 24b communicates with the reservoir tank 16 via
a low-pressure passage 28. The control fluid pressure port 24c
communicates with the STR 26 via a control fluid pressure passage 29. The
STR 26 is a two-position solenoid valve which selectively set one of the
control fluid pressure passage 29 and the high-pressure passage 22 in a
conductive state, and sets the control fluid pressure passage 29 in a
conductive state and sets the high-pressure passage 22 in a closed state
in a normal state.
A brake pedal 30 is connected to the regulator 24, and a master cylinder is
mounted to the regulator 24. The regulator 24 has a fluid pressure chamber
therein. The fluid pressure chamber always communicates with the control
fluid pressure port 24c, and selectively communicates with the
high-pressure port 24a or the low-pressure port 24b in accordance with an
operational state of the brake pedal 30. The regulator 24 is configured so
that a pressure inside the fluid pressure chamber is adjusted to a fluid
pressure corresponding to a brake pressing force FP exerted on the brake
pedal 30. Accordingly, the fluid pressure corresponding to the brake
pressing force FP always appears at the control fluid pressure port 24c of
the regulator 24. Hereinafter, this fluid pressure is referred to as a
regulator pressure PRE.
The brake pressing force FP exerted on the brake pedal 30 is mechanically
transmitted to the master cylinder 32 via the regulator 24. Additionally,
a force corresponding to the fluid pressure inside the fluid pressure
chamber of the regulator 24, that is, a force corresponding to the
regulator pressure PRE, is transmitted to the master cylinder 32.
The master cylinder 32 is provided with a first fluid pressure chamber 32a
and a second fluid pressure chamber 32b therein. A master cylinder
pressure PM/C corresponding to a resultant force of the brake pressing
force FP and a brake assist force FA is generated in the first fluid
pressure chamber 32a and the second fluid pressure chamber 32b. Both the
master cylinder pressure PM/C generated in the first fluid pressure
chamber 32a and the master cylinder pressure PM/C generated in the second
fluid pressure chamber 32b are supplied to a proportioning valve 34
(hereinafter, referred to as P valve 34).
The P valve 34 communicates with a first fluid pressure passage 36 and a
second fluid pressure passage 38. The P valve 34 supplies the master
cylinder pressure PM/C to the first fluid pressure passage 36 and the
second fluid pressure passage 38 without change in a range where the
master cylinder pressure PM/C is less than a predetermined value.
Additionally, the P valve 34 supplies the master cylinder pressure PM/C to
the first fluid pressure passage 36 without change and supplies a fluid
pressure obtained by decreasing the master cylinder pressure PM/C by a
predetermined ratio to the second fluid pressure passage 38 in a range
where the master cylinder pressure PM/C is less than a predetermined
value.
A hydraulic pressure sensor 40, which outputs an electric signal
corresponding to the master cylinder pressure PM/C, is provided between
the second fluid pressure chamber 32b of the master cylinder 32 and the P
valve 34. An output signal of the hydraulic pressure sensor 40 is supplied
to the ECU 10. The ECU 10 detects the master cylinder pressure PM/C
generated in the master cylinder 32 based on the output signal of the
hydraulic pressure sensor 40.
The above-mentioned STR 26 communicates with a third fluid pressure passage
42. The third fluid pressure passage 42 communicates with one of the
control fluid pressure passage 29 and the high-pressure passage 22 in
accordance with a state of the STR 26. In the present embodiment, wheel
cylinders 44FL and 44FR provided to left and right front wheels FL and FR
are provided with a brake fluid pressure from the first fluid pressure
passage 36 communicating with the P valve 34 or the third fluid pressure
passage 42 communicating with the STR 26. Additionally, wheel cylinders
44RL and 44RR provided to left and right rear wheels RL and RR are
provided with a brake fluid pressure from the second fluid pressure
passage 38 communicating with the P valve 34 or the third fluid pressure
passage 42 communicating with the STR 26.
The first fluid pressure passage 36 communicates with a first assist
solenoid valve 46 (hereinafter referred to as SA-146) and a second assist
solenoid valve 48 (hereinafter, referred to as SA-2). On the other hand,
the third fluid pressure passage 42 communicates with a right front
holding solenoid valve 50 (hereinafter, referred to as SFRH 50), a left
front holding solenoid valve 52 (hereinafter, referred to as SFLH 52) and
a third assist solenoid valve 54 (hereinafter, referred to as SA-354).
The SFRH 50 is a two-position solenoid valve which maintains an open state
in a normal state. The SFRH 50 communicates with the SA-146 and a right
front wheel pressure decreasing solenoid valve 58 (hereinafter, referred
to as SFRR 58) via a pressure adjusting fluid pressure passage 56. A check
valve 60 permitting a fluid flow only in a direction from the pressure
adjusting fluid pressure passage 56 to the third fluid pressure passage 42
is provided, in parallel, between the third fluid pressure passage 42 and
the pressure adjusting fluid pressure passage 56.
The SA-146 is a two-position solenoid valve which selectively renders one
of the first fluid pressure passage 36 and the pressure adjusting fluid
pressure passage 56 to be counnicated with the wheel cylinder 44FR, and
renders the first fluid pressure passage 36 and the wheel cylinder 44FR to
be in a communcating state in a normal state (OFF state). On the other
hand, the SFRR 58 is a two-position solenoid valve which renders the
pressure adjusting fluid pressure passage 56 and the reservoir tank 16 to
be in a connected state or a disconnected state. The SFRR 58 renders the
pressure adjusting fluid pressure passage 56 and the reservoir tank 16 to
be in a disconnected state in a normal state (OFF state).
The SFLH 52 is a two-position solenoid valve which maintains an open state
in a normal state. The SFLH 52 communicates with the SA-248 and a left
front wheel pressure decreasing solenoid valve 64 (hereinafter, referred
to as SFLR 64) via a pressure adjusting fluid pressure passage 62. A check
valve 66 permitting a fluid flow only in a direction from the pressure
adjusting fluid pressure passage 62 to the third fluid pressure passage 42
is provided, in parallel, between the third fluid pressure passage 42 and
the pressure adjusting fluid pressure passage 62.
The SA-248 is a two-position solenoid valve which selectively renders one
of the first fluid pressure passage 36 and the pressure adjusting fluid
pressure passage 62 to be communicated with the wheel cylinder 44FL, and
renders the first fluid pressure passage 36 and the wheel cylinder 44FL to
be in a communicating state in a normal state (OFF state). On the other
hand, the SFLR 64 is a two-position solenoid valve which renders the
pressure adjusting fluid pressure passage 62 and the reservoir tank 16 to
be in a connected state or a disconnected state. The SFLR 64 renders the
pressure adjusting fluid pressure passage 62 and the reservoir tank 16 to
be in a disconnected state from each other in a normal state (OFF state).
The second fluid pressure passage 38 communicates with the above-mentioned
SA-354. The downstream side of the SA-354 communicates with a right rear
wheel holding solenoid valve 68 (hereinafter, referred to as SRRH 68)
provided in correspondence with a wheel cylinder 44RR of the right rear
wheel RR and a left rear wheel holding solenoid valve 70 (hereinafter,
referred to as SRLR 70) provided in correspondence with a wheel cylinder
44RL of the left rear wheel RL. The SA-354 is a two-position solenoid
valve which selectively selectively renders one of the second fluid
pressure passage 38 and the third fluid pressure passage 42 to be
communicated with the SRRH 68 and the SRLR 70, and renders the second
fluid pressure passage 38, the SRRH 68 and the SRLR 70 in a communicating
state in a normal state (OFF state).
The downstream side of the SRRH 68 communicates with the wheel cylinder
44RR and a right rear wheel pressure decreasing solenoid valve 74
(hereinafter, referred to as SRRR 74) via a pressure adjusting fluid
pressure passage 72. The SRRR 74 is a two-position solenoid valve which
renders the pressure adjusting fluid pressure passage 72 and the reservoir
tank 16 in a comnunicating state or a disconnected state, and renders the
pressure adjusting fluid pressure passage 72 and the reservoir tank 16 in
the disconnected state in a normal state (OFF state). Additionally, a
check valve 76 permitting a fluid flow only in a direction from the
pressure adjusting fluid pressure passage 72 to the SA-354 is provided, in
parallel, between the SA-354 and the pressure adjusting fluid pressure
passage 72.
Similarly, the downstream side of the SRLH 70 communicates with the wheel
cylinder 44RL and a left rear wheel pressure decreasing solenoid valve 80
(hereinafter, referred to as SRLR 80) via a pressure adjusting fluid
pressure passage 78. The SRLR 80 is a two-position solenoid valve which
renders the pressure adjusting fluid pressure passage 78 and the reservoir
tank 16 in a communicating state or a disconnected state, and renders the
pressure adjusting fluid pressure passage 78 and the reservoir tank 16 in
the disconnected state In a normal state (OFF state). Additionally, a
check valve 82 permitting a fluid flow only in a direction from the
pressure adjusting fluid pressure passage 78 to the SA-354 is provided, in
parallel, between the SA-354 and the pressure adjusting fluid pressure
passage 78.
In the system according to the present embodiment, a brake switch 84 is
provided near the brake pedal 30. The brake switch 84 is a switch that
generates an ON output when the brake pedal 30 is pressed. The output
signal of the brake switch 84 is supplied to the ECU 10. The ECU 10
determines whether or not a braking operation is performed by the driver
based on the output signal of the brake switch 84.
Additionally, in the system according to the present embodiment, wheel
speed sensors 86FL, 86FR, 86RL and 86RR (hereinafter, these are referred
to as 86** as a whole) are provided near the left and right front wheels
FL and FR and the left and right rear wheels RL and RR, each of the
sensors generating a pulse signal when the respective wheel rotates a
predetermined angle. The output signals of the wheel speed sensors 86**
are supplied to the ECU 10. The ECU 10 detects a wheel speed of each of
the wheels FL, FR, RL and RR based on the output signals of the wheel
speed sensors 86**.
The ECU 10 supplies, if necessary, drive signals to the above-mentioned STR
26, SA-146, SA-248, SA-354, SFRH 50, SFLH 52, SFRR 58, SFLR 64, SRRH 68,
SRLH 70, SRRR 74 and SRLR 80 based on the output signal of the brake
switch 84.
A description will now be given of an operation of the brake force control
apparatus according to the present embodiment. The brake force control
apparatus according to the present embodiment performs the normal control
for generating a brake force corresponding to the brake pressing force FP
exerted on the brake pedal 30 when the vehicle is in a stable state. The
normal control can be achieved, as shown in FIG. 1, by turning off all of
the STR 26, SA-146, SA-248, SA-354, SFRH 50, SFLH 52, SFRR 58, SFLR 64,
SRRH 68 SRLH 70, SRRR 74 and SRLR 80 based on the output signal of the
brake switch 84.
That is, in the state shown in FIG. 1, the wheel cylinders 44FR and 44FL
communicate with the first fluid pressure passage 36, and the wheel
cylinders 44RR and 44RL communicate with the second fluid pressure passage
38. In this case, the brake fluid flows between the master cylinder 32 and
the wheel cylinders 44FR, 44FL, 44RL and 44RR (hereinafter, these may be
referred to as 44** as a whole), and a brake force corresponding to the
brake pressing force FP is generated in each of the wheels FL, FR, RL and
RR.
In the present embodiment, when a possibility for shifting to a locked
state is detected in one of the wheels, it is determined that a condition
for performing an antilock brake control (hereinafter, referred to as ABS
control) is established. The ECU 10 calculates wheel speeds VWFL, VWFR,
VWRL and VWRR (hereinafter, these are referred to as VW** as a whole) of
the wheels based on output signals of the wheel speed sensors 86**, and
calculates an assumed value VSO (hereinafter, referred to as an assumed
vehicle speed VSO) of a speed of the vehicle according to a publicly known
method. Then, when the vehicle is in a braking state, a slip rate S of
each wheel is calculated according to the following equation so as to
determine that the wheel may shift to a locked state when the slip rate S
exceeds a predetermined value.
S=(VSO-VW**).multidot.100/VSO (1)
When the condition for performing the ABS control is established, the ECU
10 outputs the drive signals to the SA-146, SA-248 and SA-354. As a
result, when the SA-146 is turned on, the wheel cylinder 44FR is
disconnected from the first fluid pressure passage 36 and connected to the
pressure adjusting fluid pressure passage 56. Additionally, when the
SA-248 is turned on, the wheel cylinder 44FL is disconnected from the
first fluid pressure passage 36 and connected to the pressure adjusting
fluid pressure passage 62. Further, when the SA-354 is turned on, the
upstream side of the SRRH 68 and the SRLH 70 is disconnected from the
second fluid pressure passage 38 and connected to the third fluid pressure
passage 42.
In this case, all wheel cylinders 44** communicate with respective holding
solenoid valves SFRH 50, SFLH 52, SRRH 68 and SRLH 70 (hereinafter, these
are referred to as holding solenoid S**H) and respective pressure
decreasing solenoid valves SFRR 58, SFLR 64, SRRR 74 and SRLR 80
(hereinafter, these are referred to as pressure decreasing solenoid S**R),
and a regulator pressure PRE is introduced to the upstream side of each of
the holding solenoids S**H via the third fluid pressure passage 42 and the
STR 26.
In the above-mentioned condition, a wheel cylinder pressure PW/C of the
respective wheel cylinders 44** is increased with the regulator pressure
PRE as an upper limit by the holding solenoids S**H being in an open state
and the pressure decreasing solenoids S**R being in a closed state.
Hereinafter, this state is referred to as a pressure increasing mode 1.
Additionally, the wheel cylinder pressure PW/C of the respective wheel
cylinders 44** is maintained without being increased or decreased by the
holding solenoids S**H being in a closed state and the pressure decreasing
solenoids S**R being in the closed state. Hereinafter, this state is
referred to as a holding mode 2. Further, the wheel cylinder pressure PW/C
of the respective wheel cylinders 44** is decreased by the holding
solenoids S**H being in the closed state and the pressure decreasing
solenoids S**R being in the open state. Hereinafter, this state is
referred to as a pressure decreasing mode 3. The ECU 10 achieves, if
necessary, the above-mentioned pressure increasing mode 1, holding mode 2
and pressure decreasing mode 3 so that a slip rate S of each wheel during
a braking time becomes an appropriate value, that is, so that each wheel
does not shift to the locked state.
When a depression of the brake pedal 30 is released by the driver during
execution of the ABS control, the wheel cylinder pressure PW/C must be
immediately decreased. In the system according to the present embodiment,
the check valves 60, 66, 76 and 82 are provided in hydraulic pressure
paths corresponding to each of the wheel cylinders 44**, each of the check
valves 60, 66, 76 and 82 permitting a fluid flow only in the directions
from the wheel cylinders 44** to the third fluid pressure passage 42.
Thus, according to the system of the present embodiment, the wheel
cylinder pressures PW/C of all of the wheel cylinders 44** can be
immediately decreased after the depression of the brake pedal 30 is
released.
In the system according to the present embodiment, when the ABS control is
performed, the wheel cylinder pressure PW/C is increased by the brake
fluid being supplied from the regulator 24 to the wheel cylinders 44**,
that is, by the brake fluid being supplied from the pump 12 to the wheel
cylinders 44**, and is decreased by the brake fluid in the wheel cylinders
44** flowing to the reservoir tank 16. When the increase in the wheel
cylinder pressure PW/C is performed by using the master cylinder 32 as a
fluid pressure source and if the pressure increasing mode and the pressure
decreasing mode are repeatedly performed, the brake fluid in the master
cylinder 32 gradually decreases and a so-called bottoming of the master
cylinder may occur.
On the other hand, if the pump 12 is used as a fluid pressure source so as
to increase the wheel cylinder pressure PW/C, as in the system according
to the present embodiment, such a bottoming can be prevented. Thus, in the
system according to the present embodiment, a stable operational state can
be maintained if the ABS control is continued for a long time.
In the system according to the present embodiment, the ABS control is
started when a possibility for shifting to the locked state is detected in
one of the wheels. Accordingly, in order to start the ABS control, as a
precondition, a braking operation having a level at which a large slip
rate S is generated in one of the wheels must be performed.
FIG. 2 shows changes in the brake pressing force FP applied to the brake
pedal 30 with respect to time under various conditions. Curves indicated
by 1 and 2 in FIG. 2 represent changes in the pressing force FP when an
emergency braking is performed by a highly skilled driver (hereinafter,
referred to as a high-grade driver) and an unskilled driver or a driver
lacking (hereinafter, referred to as a beginner-grade driver),
respectively. The emergency braking operation is an operation performed
when is it desired to rapidly decelerate a vehicle. Accordingly, the brake
pressing force associated with the emergency braking operation is
preferably a force sufficiently large as the ABS control is performed.
As shown by the curve 1, when the driver of the vehicle is a high-grade
driver, the brake pressing force FP is immediately and rapidly increased
in response to establishment of a condition in which an emergency braking
is required, and a large brake pressing force FP can be maintained for a
long time. If such a brake pressing force FP is exerted on the brake pedal
30, a sufficiently high brake fluid pressure can be provided from the
master cylinder 32 to each of the wheel cylinders 44** so as to start the
ABS control.
However, as shown by the curve 2 when the driver of the vehicle is a
beginner-grade driver, the brake pressing force FP may not be increased to
a sufficiently high value in response to the condition in which an
emergency braking is required. If the brake pressing force FP exerted on
the brake pedal 30 is not sufficiently increased as shown by the curve 2
after an emergency braking is required, the wheel cylinder pressure PW/C
in each of the wheels 44** is not sufficiently increased, which results in
a possibility that the ABS control is not started.
As mentioned above, when the driver of the vehicle is a beginner-grade
driver, the braking ability of the vehicle may not be sufficiently
performed even when an emergency braking operation is performed despite
that the vehicle has a good braking ability. Accordingly, the system
according to the present embodiment is provided with a brake assist
function for sufficiently increasing the wheel cylinder pressure PW/C even
if the brake pressing force FP is not sufficiently increased when the
brake pedal is operated with an intention to perform an emergency braking.
Hereinafter, a control performed by the ECU 10 to achieve such a function
is referred to as a brake assist control.
In the system according to the present embodiment, when performing the
brake assist control, an accurate determination must be made as to
whether, when the brake pedal 30 is operated, the operation is intended to
perform an emergency braking operation or to perform a regular braking
operation.
Curves indicated by shown 3 and 4 in FIG. 2 show changes in the brake
pressing force FP when the driver operates the brake pedal with an
intention to perform a normal braking operation under various conditions.
As shown by the curves 1 to 4, a change in the brake pressing force FP
associated with the normal braking operation is gentle as compared to a
change in the brake pressing force FP associated with an emergency braking
operation. Additionally, a convergent value of the brake pressing force FP
associated with the normal braking operation is not so large as a
convergent value of the brake pressing force FP associated with an
emergency braking operation.
Giving attention to those differences, when the brake pressing force FP is
increased to a sufficiently large value at a rate of change exceeding a
predetermined value after a braking operation is started, that is, when
the brake pedal 30 is operated so that the brake pressing force FP reaches
an area indicated by (I) in FIG. 2, it can be determined that an emergency
braking is performed.
Additionally, when the rate of change of the brake pressing force FP is
smaller than the predetermined value or when the convergent value of the
brake pressing force FP is smaller than the predetermined value, that is,
when the brake pedal 30 is operated so that the brake pressing force FP
always changes within an area indicated by (II) in FIG. 2, it can be
determined that a normal braking operation is performed.
Accordingly, in the system according to the present embodiment, an
operational speed and an amount of operation of the brake pedal are
detected or assumed, and, then, it is determined whether or not the
operational speed exceeds a predetermined value and whether or not the
amount of operation exceeds a predetermined value, and, thereby, it can be
determined whether or not the operation on the brake pedal 30 is intended
to perform an emergency braking.
In the brake force control apparatus according to the present embodiment,
the brake pedal 30 is moved by an increase or decrease in the brake
pressing force FP. At this time, a larger operational speed is generated
in the brake pedal 30 as the brake pressing force shows a steep slope, and
an amount of operation substantially corresponding to the brake pressing
force FP is generated. Accordingly, the operational speed and the amount
of operation of the brake pedal 30 can be accurately assumed from the
brake pressing force FP.
When the brake pressing force FP is exerted on the brake pedal 30. a stroke
L corresponding to the brake pressing force FP is generated in the brake
pedal 30. Additionally, when the stroke L is generated in the brake pedal
30, a master cylinder pressure PM/C corresponding to the stroke L, which
corresponds to the brake pressing force FP is generated in the master
cylinder 32. When the master cylinder pressure PM/C corresponding to the
brake pressing force FP is generated, a vehicle deceleration G
corresponding to the brake pressing force FP is generated in the vehicle.
Accordingly, an operational speed and an amount of operation of the brake
pedal 30 can be assumed from parameters including 2+L the pedal stroke L,
3+L the master cylinder pressure PM/C, 4+L the vehicle deceleration G,
5+L the assumed vehicle speed VSO and 6+L the wheel speed Vw**, other
than the above-mentioned By brake pressing force FP.
In order to accurately assume an operational speed and an amount of
operation of the brake pedal 30, that is, in order to accurately
discriminate an emergency braking and a normal brake, preferred parameters
of the above-mentioned parameters (hereinafter, referred to as basic
parameters) are those detected at positions closest to the foot of the
driver. According to such a point of view, the parameters 1+L to 6+L
have a superiority in the order of 1.fwdarw.6 when used as the basic
parameters.
In order to detect 1+L the brake pressing force FP, it is required to
provide (i) a pressing force sensor. Additionally, in order to detect 2+L
the pedal stroke L, it is required to provide (ii) a stroke sensor.
Similarly, in order to detect 3+L the master cylinder pressure PM/C and
4+L the vehicle deceleration G, it is required to provide a (iii) a
hydraulic pressure sensor and (iv) a deceleration sensor, respectively.
Further, in order to detect 5+L the assumed vehicle speed VSO and 6+L
the wheel speed VW**, it is required to provide (v) a wheel speed sensor.
The (v) wheel speed sensor and the (iv) deceleration sensor among the
above-mentioned sensors (i) to (v) are conventionally and widely used
sensors for a vehicle. On the other hand, the (ii) stroke sensor and the
(i) pressing force sensor are not popular sensors for a vehicle.
Accordingly, considering a cost merit of a sensor due to a mass production
effect, the above-mentioned sensors (i) to (v) have a superiority in the
order of (v).fwdarw.(i).
In the system according to the present embodiment, considering the
above-mentioned merit and demerit, the hydraulic pressure sensor 40 is
used as a sensor for detecting the basic parameters so as to discriminate
an emergency braking operation and a normal braking operation by using the
master cylinder pressure PM/C as a basic parameter. A description will now
be given of an operation of the system according to the present embodiment
when it Is determined by the ECU 10 that an emergency braking is
performed.
The ECU 10 determines that an emergency braking is performed when the
master cylinder pressure PM/C exceeding the predetermined value is
detected and a rate of change .DELTA.PM/C is detected after the brake
pedal 30 is pressed. When it Is determined that an emergency braking is
performed, the ECU 10 outputs the drive signals to the STR 26, the SA-146,
the SA-248 and the SA-354.
When the STR 26 is turned on upon receipt of the above-mentioned drive
signal, the third fluid pressure passage 42 and the high-pressure passage
22 are directly connected to each other. In this case, an accumulator
pressure PACC is introduced into the third fluid pressure passage 42.
Additionally, when the SA-146 and the SA-248 are turned on upon receipt of
the drive signals, the wheel cylinders 44FR and 44FL communicate with the
pressure adjusting fluid pressure passages 56 and 62, respectively.
Further, when the SA-354 is turned on upon receipt of the above-mentioned
drive signal, the upstream side of the SRRH 68 comminicates with the third
fluid pressure passage 42. In this case, a state is established in which
all of the wheel cylinders 44** communicate with the respective holding
solenoids S**H and the respective pressure decreasing solenoids S**R and
the accumulator pressure PACC is introduced to the upstream side of each
of the holding solenoids S**H.
In the ECU 10, all of the holding solenoids S**H and all of the pressure
decreasing solenoids S**R are maintained in the OFF state immediately
after execution of an emergency braking is detected. Accordingly, as
mentioned above, when the accumulator pressure PACC is introduced to the
upstream side of the holding solenoids S**H, the fluid pressure is
provided to the wheel cylinders 44** without being changed. As a result,
the wheel cylinder pressure PW/C of all of the wheel cylinders 44** is
increased toward the accumulator pressure PACC.
As mentioned above, according to the system of the present embodiment, when
an emergency braking is performed, the wheel cylinder pressure PW/C of all
of the wheel cylinders 44** can be imnediately increased irrespective of a
magnitude of the brake pressing force FP. Thus, according to the system of
the present embodiment, a large brake force can be generated immediately
after establishment of a condition in which an emergency braking is
required, even if the driver is a beginner-grade driver.
When the accumuator pressure PACC begins to be supplied to the wheel
cylinders 44**, as mentioned above, a slip rate S of each of the wheels
FL, FR, RL and RR is rapidly increased, and the condition for performing
the ABS control is finally established. When the condition for performing
the ABS control is established, the ECU 10 achieves, if necessary, the
above-mentioned pressure increasing mode 1, holding mode 2 and pressure
decreasing mode 3 so that the slip rate S of each of the wheels becomes an
appropriate value, that is, so that each of the wheels does not shift to
the locked state.
It should be noted that when the ABS control is performed subsequent to an
emergency braking operation, the wheel cylinder pressure PW/C is increased
by using the pump 12 and the accumulator 20 as a fluid pressure source,
and is decreased by the brake fluid in the wheel cylinders 44** flowing to
the reservoir tank 16. Accordingly, if the pressure increasing mode and
the pressure decreasing mode are repeated, a so-called bottoming of the
master cylinder 32 does not occur.
When the brake assist control is started as mentioned above by execution of
an emergency braking operation, the brake assist control must be ended
when a press of the brake pedal 30 is released. In the system according to
the present invention, as mentioned above, the STR 26, the SA-146, the
SA-248 and the SA-354 are maintained to be in the ON state. When the STR
26, the SA-146, the SA-248 and the SA-354 are in the ON state, each of the
fluid pressure chamber in the regulator 24 and the first fluid pressure
chamber 32a and the second fluid pressure chamber 32b becomes
substantially a closed space.
In this case, the accumulator pressure PACC is supplied to the wheel
cylinder of each of the wheels but the master cylinder pressure PM/C
corresponding to the brake pressing force FP is supplied to the hydraulic
pressure sensor 40. Accordingly, the ECU can accurately determine whether
or not the press of the brake pedal 30 is released based on the detected
value of the hydraulic pressure sensor 40. When the release of the press
of the brake pedal 30 is detected, the ECU 10 stops the supply of the
drive signals to the STR 26, the SA-146, the SA-248 and the SA-354 so as
to return the brake force control apparatus to a state (hereinafter,
referred to as a normal brake state) in which the normal control is
performed.
As for the basic parameters which are the basis of discrimination between
an emergency braking and a normal brake, 1+L the brake pressing force FP,
2+L the pedal stroke L, 4+L the vehicle deceleration G, 5+L the assumed
vehicle speed VSO and 6+L the wheel speed VW** other than the
above-mentioned 33+L master cylinder pressure PM/C may be applicable.
Among those parameters, the 1+L brake pressing force FP and 2+L the
pedal stroke L are parameters that are sensitive to a change in the brake
pressing force FP, similar to 3+L the master cylinder pressure PM/C.
Accordingly, when 1+L the brake pressing force FP or 2+L the pedal
stroke L is used as a basic parameter, it can be easily determined whether
or not the press of the brake pedal 30 is released by monitoring the
parameter.
On the other hand, 4+L the vehicle deceleration G and 6+L the wheel speed
VW** are parameters that are changed by a change in a brake force. In
other words, during execution of the brake assist control, the brake
pressing force FP is hardly reflected in those parameters. Accordingly,
when the parameters of 4+L to 6+L are used as the basic parameter, it is
effective to perform a determination for a termination of the brake assist
control based on the output state of a pressing force switch that is
provided for outputting different signals according to whether the brake
pressing force FP is applied or released.
An apparatus, such as the brake force control apparatus according to the
present embodiment, which generates a brake force larger than that of a
normal braking operation when an emergency braking operation is performed
is effective for providing a superior braking ability to the vehicle when
the driver is a beginner-grade driver. However, in such an apparatus, it
is important to achieve the above-mentioned functions without giving an
incongruous feel to the driver. The brake force control apparatus
according to the present embodiment has a feature in that the brake assist
control can be started without giving an incongruous feel to the driver by
changing the condition for performing the brake assist control, if
necessary, in accordance with a state of the vehicle or an operational
state of the brake pedal 30.
A description will now be given, with respect to FIG. 3, of contents of a
process performed by the ECU 10 to achieve the above-mentioned functions.
FIG. 3 shows a map of start conditions of the brake assist control used by
the ECU 10. The start conditions (I), (II) and (III) shown in FIG. 3 can
be represented as follows.
P1<PM/C and .DELTA.P2<.DELTA.PMC<.DELTA.P4 (I)
P2<PM/C and .DELTA.P1<.DELTA.PM/C <.DELTA.P3 (II)
P2<PM/C and .DELTA.P2<.DELTA.PM/C<.DELTA.P3 (III)
The ECU 10 selects an optimum condition from among the above mentioned
start conditions (I) to (III) in accordance with the assumed vehicle speed
VSO and an elapsed time T after the brake pedal 30 is pressed so as to
start the brake assist control when the master cylinder pressure PM/C and
the rate of change .DELTA.PM/C satisfy the selected condition.
As mentioned above, each of the start conditions (I) to (III) used in the
present embodiment is two-dimensionally set according the master cylinder
pressure PM/C and the rate of change .DELTA.PM/C. Accordingly, if any one
of the start conditions is used, the brake assist control is not started
by the brake pedal 30 being slightly operated at a high-speed, that is,
the brake pedal 30 being slightly pressed at a high-speed. Thus, according
to the brake force control apparatus of the present embodiment, when the
driver operates the brake pedal 30 at a high-speed without an intention to
rapidly decelerate the vehicle, the brake assist control is prevented from
being erroneously started.
In the system according to the present embodiment, there is a certain time
delay until the wheel cylinder pressure PW/C begins to be increased by
execution of the brake assist control after an emergency braking is
detected. Therefore, when the master cylinder pressure PM/C is increased
at a high speed, the master cylinder pressure PM/C can be rapidly
increased by continuing the normal control rather than starting the brake
assist control.
In each of the start conditions (I) to (III) used in the present
embodiment, an upper limit value with respect to the rate of change
.DELTA.PM/C of the master cylinder pressure PM/C is set. Accordingly, even
if any one of the start conditions is used, the brake assist control is
not started when the driver is a high-grade driver and the master cylinder
pressure PM/C is increased at a sufficiently high speed.
Thus, according to the brake force control apparatus of the present
embodiment, a brake force can be rapidly increased by performing the brake
assist control when the driver is a beginner-grade driver. Additionally,
when the driver is a high-grade driver, the brake force can be rapidly
raised by prohibiting execution of the brake assist control.
The ECU 10 selects the start condition (I) or (II) when the assumed vehicle
speed VSO is greater than a predetermined speed VH, that is, when the
vehicle is moving at a high or middle speed. On the other hand, the ECU 10
selects the start condition (III) when the assumed vehicle speed VSO is
smaller than the predetermined speed VH, that is, when the vehicle is
moving at a low speed.
A deceleration feel given to the driver when a full-braking is performed in
the vehicle is smaller as the vehicle moves faster, and is larger as the
vehicle moves slower. Accordingly, if the brake assist control is
performed at a frequency similar to that of the vehicle moving at a high
speed when the vehicle is moving at a low speed, a riding quality at a low
speed is deteriorated.
In the present embodiment, the start condition (III) which is selected when
the vehicle is moving at a high speed is narrower and harder to establish
as compared to the start condition (I) or (II) which is selected when the
vehicle is moving at a middle or high speed. Thus, according to the brake
force control apparatus of the present embodiment, when the vehicle is
moving at a low speed, the brake assist control is hardly started as
compared to a case in which the vehicle is moving at a middle or high
speed. Therefore, according to the brake force control apparatus of the
present embodiment, both a superior braking ability and a superior riding
quality can be obtained during the entire vehicle speed area.
Additionally, the ECU 10 selects the start condition (I) immediately after
the brake pedal 30 is pressed. On the other hand, the ECU 10 selects the
start condition after a predetermined period T0passes after the brake
pedal 30 is pressed during the middle or high-speed movement.
When the driver presses the brake pedal 30 with an intention to perform an
emergency from the beginning, the master cylinder pressure PM/C and the
rate of change .DELTA.PM/C thereof start to rapidly increase immediately
after the brake pedal 30 is pressed. Accordingly, considering such a
condition, it is appropriate to determine whether or not the braking
operation being performed is an emergency braking operation based on PM/C
and .DELTA.PM/C obtained immediately after the brake pedal is pressed.
Additionally, when an emergency braking is intended from the beginning as
mentioned above, the master cylinder pressure PM/C starts to increase from
an atmospheric pressure. In this case, the master cylinder pressure PM/C
shows a rapid increase in a relatively low-pressure area. Accordingly, in
such a case, a threshold value with respect to the master cylinder
pressure PM/C should be set to a relatively small value and a threshold
value with respect to the rate of change .DELTA.PM/C should be set to a
relatively large value.
On the other hand, if the driver intends to perform an emergency braking
after the brake pedal has been pressed, the master cylinder pressure PM/C
and the rate of change .DELTA.PM/C start to increase after a certain
period passes after the brake pedal is pressed. Thus, a determination
should be made that an emergency braking is intended to be performed after
the brake pedal 30 is pressed when the master cylinder pressure PM/C
starts to increase after a predetermined time T0has been passed after the
brake pedal is pressed.
As mentioned above, when an emergency braking is intended to be performed
after the brake pedal 30 was pressed, the master cylinder pressure PM/C is
further increased after increasing to a certain level. In this case, the
master cylinder pressure PM/C shows a rapid increase in a relatively
high-pressure area. However, in such a condition, the rate of change
.DELTA.PM/C as large as that generated when the master cylinder pressure
PM/C is increased from an atmospheric pressure is not generated.
Accordingly, in such a case, the threshold value with respect to the
master cylinder pressure PM/C should be set to a relatively large value
and the threshold value with respect to the rate of change .DELTA.PM/C
should be set to a relatively small value.
In the present embodment, the start conditions (I) and (II) are set so as
to satisfy the above-mentioned conditions, respectively. Thus, according
to the brake force control apparatus of the present embodiment, the brake
assist control can be started along with the driver's intention both when
the brake pedal 30 is pressed with an intention to perform an emergency
braking from the beginning and when an emergency braking is intended to be
performed after the brake pedal 30 is pressed.
FIG. 4 is a flowchart of an example of a control routine performed by the
ECU 10. It should be noted that the routine shown in FIG. 4 is a periodic
interruption routine which is started at every predetermined time. When
the routine shown in FIG. 4 is started, the process of step 100 is
performed first.
In step 100, it is determined whether or not the master cylinder pressure
PM/C is larger than a predetermined value .alpha.. The predetermined value
.alpha. is a value which is not output when the hydraulic pressure sensor
40 is normally operated. Accordingly, if it is determined that
PM/C>.alpha. is established, it can be determined that an abnormality
occurs in the hydraulic pressure sensor 40. In this case, the process of
step 102 is performed subsequently. On the other hand, if it is determined
that PM/C>.alpha. is not established, the process of step 104 is
performed.
In step 102, execution of the brake assist control is prohibited.
Accordingly, when an abnormality occurs in the hydraulic pressure sensor
40, the control is not continued based on an abnormal master cylinder
pressure PM/C. After the process of step 102 is completed, the routine at
this time is ended.
In step 104, it is determined whether or not the rate of change .DELTA.PM/C
of the master cylinder pressure PM/C is greater than a predetermined value
.beta.. The predetermined value .beta. is a value which is not generated
when the hydraulic pressure sensor 40 normally outputs the master cylinder
pressure PM/C. Accordingly, if it is determined that .DELTA.PM/C>.beta. is
established, it can be determined that a noise is superimposed on the
output signal of the hydraulic pressure sensor 40. In this case, the
process of step 102 is performed subsequently. Thus, according to the
brake force control apparatus of the present embodiment, an improper
control is not performed due to an influence of a noise. On the other
hand, if it is determined that .DELTA.PM/C>.beta. is not established, the
process of step 106 is performed next.
In step 106, it is determined whether or not the assumed vehicle speed VSO
is greater than the predetermined speed VH. As a result, if it is
determined that VSO.gtoreq.VH is established, it can be determined that
the vehicle is moving at a middle or high speed. In this case, the process
of step 108 is performed next.
In step 108, it is determined whether or not the elapsed time T after the
brake pedal 30 is pressed, that is, after an ON signal starts to be output
from the brake switch 84 is smaller than the predetermined time T0. As a
result, if it is determined that T <T0is established, the process of step
110 is performed so as to proceed with the process by using the start
condition (I) shown in FIG. 3. On the other hand, if it is determined that
T <T0is not established, the process of step 112 is performed so as to
proceed with the process by using the start condition (II) shown in FIG.
3.
In step 110, it is determined whether or not the master cylinder pressure
PM/C and the rate of change .DELTA.PM/C thereof satisfy the start
condition (I), that is, it is determined whether or not P1<PM/C and
.DELTA.P2<.DELTA.PM/C<.DELTA.P4are established. As a result, if the
above-mentioned condition is established, it is determined that an
emergency braking operation is performed by the driver, and, then, the
process of step 114 is performed. On the other hand, if the
above-mentioned condition is not established, the process is not continued
and the routine at this time is ended.
In step 114, execution of the brake assist control is started. Thereafter,
the brake assist control is continued until the press of the brake pedal
30 is released and the master cylinder pressure PM/C is decreased. After
the process of step 114 is ended, the routine at this time is ended.
In step 112, it is determined whether or not the master cylinder pressure
PM/C and the rate of change .DELTA.PM/C thereof satisfy the start
condition (II), that is, it is determined whether or not P2<PM/C and
.alpha.P1<.DELTA.PM/C<.alpha.P3are established. As a result, if the
above-mentioned condition is established, it is determined that an
emergency braking operation is performed by the driver, and, then, the
process of step 114 is performed. On the other hand, if the
above-mentioned condition is not established, the process is not continued
and the routine at this time is ended.
If it is determined, in step 106, that the assumed vehicle speed VSO is
lower than the predetermined speed VH, it is then determined, in step 116,
whether or not the assumed vehicle speed VSO is greater than a
predetermined speed VL (<VH). The brake assist control is a process for
rapidly decelerating a vehicle. Accordingly, if the vehicle can be easily
stopped without performing such a control, the brake assist control is not
necessarily performed. The predetermined speed VL is a minimum speed of
the vehicle at which the brake assist control can provide a merit.
Accordingly, if it is determined that VSO.gtoreq.VL is not established, it
can be determined that the brake assist control is not needed to be
performed. In this case, the routine at this time is ended without
performing any process thereafter. On the other hand, if it is determined
that VSO 2 VL is established, the step of 118 is performed next.
In step 118, it is determined whether or not the master cylinder pressure
PM/C and the rate of change .DELTA.PM/C thereof satisfy the start
condition (II), that is, it is determined whether or not P2<PM/C and
.alpha.P2<.DELTA.PM/C<.alpha.P3are established. As a result, if the
above-mentioned condition is established, it is determined that an
emergency braking operation is performed by the driver, and, then, the
process of step 114 is performed. On the other hand, if the
above-mentioned condition is not established, the process is not continued
and the routine at this time is ended.
In the above-mentioned embodiment, when setting the start conditions (I) to
(III), although an upper limit value is provided to only the rate of
change .DELTA.PM/C, the present invention is not limited to this and an
upper limit value may be provided to a condition of the master cylinder
pressure PM/C.
As mentioned above, according to the routine shown in FIG. 4, the brake
assist control is continued until the master cylinder pressure PM/C is
decreased after the condition for execution of the brake assist control is
established. However, depending on moving circumstances of the vehicle,
the braking operation may reach an area where the brake assist is not
needed after a braking operation satisfying the condition for executing
the above-mentioned (I) to (III). In such a case, it is appropriate to
restart the normal control by ending the brake assist control so as to
maintain a sufficiently large brake force without giving an incongruous
feel to the driver.
Accordingly, in the brake force control apparatus of the present
embodiment, the master cylinder pressure PM/C and the rate of change
.DELTA.PM/C are continuously monitored after the brake assist control is
started so that the execution of the brake assist control is canceled when
it is determined that the braking operation by the driver has reached an
area in which a response can be made to a request for an emergency
braking.
FIG. 5 is a flowchart of an example of a control routine performed by the
ECU 10 so as to achieve the above-mentioned function. The routine shown in
FIG. 5 is a periodic interruption routine which is started at every
predetermined time. When the routine shown in FIG. 5 is started, the
process of step 120 is performed first.
In step 120, it is determined whether or not the brake assist control is
being performed. This routine is a routine for canceling an execution of
the brake assist control under a predetermined condition. Accordingly, if
the brake assist control is not being performed, there is no merit to
continue the subsequent process. Thus, if it is determined that the brake
assist control is not being performed, the process is not continued and
the routine at this time is ended. On the other hand, if it is determined
that the brake assist control is being performed, the process of step 122
is performed next.
In step 122, it is determined whether or not the master cylinder pressure
PM/C exceeds a predetermined threshold value A. As a result, if it is
determined that PM/C>A is established, the process of step 124 is
performed.
In step 124, it is determined whether or not the rate of change .DELTA.PM/C
of the master cylinder pressure PM/C exceeds a predetermined threshold
value B. The above-mentioned threshold values A and B are threshold values
which are set so as to determine whether or not the braking operation by
the driver has reached the area in which the brake assist control is
unnecessary. Accordingly, if it is determined, in step 122, that PM/C>A is
established, or if it is determined, in step 124, that .DELTA.PM/C>B is
established, it can be determined that a condition in which the brake
assist control is not necessarily performed is established. In such cases,
the process of step 126 is performed subsequently.
In step 126, a process for canceling the execution of the brake assist
control is performed. Specifically, a process of turning off the STR 26,
the SA-146, SA-248 and SA-354 is performed. It should be noted that after
the process of step 126 is completed, the routine at this time is ended.
After the above-mentioned process is performed, the wheel cylinders 44**
communicate with the master cylinder 32, and the normal control is
restarted.
On the other hand, if both the condition of step 122 and the condition of
step 124 are not established, it can be determined that the condition in
which the brake assist control is required is maintained. In this case,
the routine at this time is ended without performing any process.
According to the above-mentioned process, the execution of the brake assist
control, which has been started, can be canceled when the emergency
braking operation is changed to a sharp operation after it is started with
a gentle change. Thus, according to the brake force control apparatus of
the present embodiment, a good operational feel which does not give an
incongruous feel to a driver can be achieved.
A description will now be given, with reference to FIG. 6 and FIG. 7, of a
second embodiment according to the present invention. FIG. 6 shows a
system structure diagram of a brake force control apparatus according to
the present invention. It should be noted that, in FIG. 6, only a part of
the brake force control apparatus corresponding to one wheel is shown for
the sake of convenience.
The brake force control apparatus shown in FIG. 6 is controlled by an ECU
200. The brake force control apparatus according to the present embodiment
has a brake pedal 202. A brake switch 203 is provided near the brake pedal
202. The brake switch 203 is a switch which generates an ON output when
the brake pedal 202 is pressed. The output signal of the brake switch 203
is supplied to the ECU 200. The ECU 200 determines whether or not a
braking operation is being performed based on the output signal of the
brake switch 203.
The brake pedal 202 is connected to a vacuum booster 204. The vacuum
booster 204 is an apparatus which assists a brake pressing force by using
an intake negative pressure of an internal combustion engine as a power
source. The brake force control apparatus according to the present
embodiment has a feature to generate an assist power having a
predetermined power ratio with respect to a brake pressing force FP when a
normal braking operation is performed, and generate a maximum assist power
irrespective of the brake pressing force FP when an emergency braking is
performed. A structure of the vacuum booster 204 will be described later.
A master cylinder 206 is fixed to the vacuum booster 204. The master
cylinder 206 has a fluid pressure chamber therein. Additionally, a
reservoir tank 208 is provided above the master cylinder 206. The fluid
pressure chamber of the master cylinder and the reservoir tank 208
communicate with each other when a press of the brake pedal 202 is
released, whereas they are disconnected from each other when the brake
pedal is pressed. Accordingly, brake fluid is supplied to the fluid
pressure chamber each time the press of the brake pedal 202 is released.
The fluid pressure chamber of the maser cylinder 206 communicates with a
fluid pressure passage 210. The fluid pressure passage 210 is provided
with a hydraulic pressure sensor 212 which outputs an electric signal
corresponding to a pressure inside the fluid pressure passage 210. The
output signal of the hydraulic pressure sensor 212 is supplied to the ECU
200. The ECU 200 detects a fluid pressure generated by the master cylinder
206, that is, the master cylinder pressure PM/C based on the output signal
of the hydraulic pressure sensor 212.
The fluid pressure passage 210 is provided with a holding solenoid 216
(hereinafter, referred to as SH 216). The SH 216 is a two-position
solenoid valve which maintains an open state in a normal state (OFF
state). The SH 216 is set to be in an ON state (closed state) by a drive
signal being supplied by the ECU 200.
The downstream side of the SH 216 commuicates with a wheel cylinder 218 and
a pressure decreasing solenoid 220 (hereinafter, referred to as SR220).
The SR 220 is a two-position solenoid valve which maintains a closed state
in a normal state (OFF state). SR 220 is set to be in an ON state (open
state) by a drive signal being supplied by the ECU 200. Additionally, a
check valve 222 which permits a fluid flow only in a direction from the
wheel cylinder 218 to the fluid pressure passage 210 is provided between
the wheel cylinder 218 and the fluid pressure passage 210.
It should be noted that a wheel speed sensor 219 which generates a pulse
signal each time the wheel rotates a predetermined angle is provided near
the wheel cylinder 218. An output signal of the wheel speed sensor 219 is
supplied to the ECU 200. The ECU 200 detects a wheel speed based on the
output signal of the wheel speed sensor 219.
A reservoir 224 is provided on the downstream side of the SR 220. The brake
fluid flowing out of the SR 220 when the SR 220 is set to be in the ON
state (open state) is stored in the reservoir 224. It should be noted that
the reservoir previously stores a predetermined amount of brake fluid. The
reservoir 224 communicates with an inlet port 226a of a pump 226.
Additionally, an outlet port 226b of the pump 226 communicates with the
fluid pressure passage 210 via a check valve 228. The check vale 228 is a
one-way valve which permits a fluid flow only in a direction from the pump
226 to the fluid pressure passage 210.
A description will now be given of a structure of the vacuum booster 204
and a structure of a periphery thereof. FIG. 7 shows a structure of the
vacuum booster 204 and a structure of a periphery thereof. It should be
noted that, in FIG. 7, the master cylinder 206 is fixed to the vacuum
booster 204 on the left side thereof. Additionally, the brake pedal 202 is
connected to the vacuum booster 204 on the right side thereof.
The vacuum booster 204 includes a housing 234 which comprises a front shell
230 and a rear shell 232. A diaphragm 236 and a cylinder member 238 are
provided inside the housing 234. The cylinder member 238 is a cylindrical,
elastic member having a side surface formed in bellows so that the
cylinder member can be elongated and compressed in leftward and rightward
directions in FIG. 7. An inner space of the housing 234 is divided into a
negative pressure camber 240, a first pressure changing chamber 242 and a
second pressure changing chamber 244 by the diaphragm 236 and the cylinder
member 238.
The front shell 230 is provided with a negative pressure introducing port
246 which communicates with the negative pressure chamber 240. The
negative pressure introducing port 246 commicates with a negative pressure
passage 248 which communicates with a negative pressure source such as,
for example, an intake passage of an internal combustion engine. The front
shell 230 is also provided with a adjusting pressure introducing port 250
which communicates with the second pressure changing chamber 244. The
adjusting pressure introducing port 250 communicates with a negative
pressure introducing valve 252 and an adjusting pressure passage 256 which
is communicated to an atmospheric pressure introducing valve 254.
The negative pressure introducing valve 252 is a two-position solenoid
valve which is positioned between the adjusting pressure passage 256 and
the negative pressure passage 248, and maintains an open state in a normal
state (OFF state). On the other hand, the atmospheric pressure introducing
valve 254 is a two-position solenoid valve which controls communication
between the adjusting pressure passage 256 and an atmosphere, and
maintains a closed state in a normal state (OFF state). The negative
pressure introducing valve 252 and the atmospheric pressure introducing
valve 254 are rendered to be in the ON state (closed state or open state,
respectively) by a drive signal being supplied by the ECU 200.
The rear shell 232 is provided with an atmospheric pressure introducing
port 258 which communicates with the first pressure changing chamber 242.
The atmospheric pressure introducing port 258 communicates with the
adjusting pressure passage 256 via a check valve 260. The check valve 260
is a one-way valve which permits a fluid flow only in a direction from the
adjusting pressure passage 256 to the atmospheric pressure introducing
port 258. Accordingly, air flows through the atmospheric pressure
introducing port 258 only when a pressure higher than a pressure in the
first pressure changing chamber 242 is generated in the adjusting pressure
passage 256.
A booster piston 262 is fit in the center of the diaphragm 236. The booster
piston 262 is slidably supported by the rear shell 232 so that an end
thereof is exposed in the second pressure-changing chamber 244.
Additionally, the booster piston 262 is urged toward an original position,
that is, in a rightward direction in FIG. 7, by a spring 263 provided
within the second pressure-changing chamber 244.
An inner space 264 is formed in a center of the booster piston 262, the
inner space extending in a radial direction of the booster piston 262.
Additionally, the booster piston 262 is provided with a negative pressure
passage which connects the second pressure changing chamber 244 to the
internal space 264 and a pressure changing passage 268 which connects the
internal space 264 and the first pressure changing chamber 242.
The internal space 264 of the booster piston 262 is provided with a
pressing force transmitting member 270 which is slidable in an axial
direction thereof. The pressing force transmitting member 210 has an
annular air valve 272 on an end located on a rearward side of the vehicle,
and has a cylindrical pressing force transmitting part 274 on an end
located on a forward side of the vehicle.
A control valve 276 is provided in the internal space 264 of the booster
piston 262. The control valve 276 includes a cylindrical part 278 fixed on
an inner wall of the internal space 264 and a flat part 280 formed on an
end located on a forward side of the vehicle. The flat portion 280 can
move inside the inner space 264 in an axial direction of the control valve
276 with elongation and compression of the cylinder part 278.
A through hole 282 is formed in the flat portion 280 of the control valve
276, the through hole 282 extending in the center of the flat portion 280.
An input rod 284 is inserted into the through hole 282. The diameter of
the through hole 282 is sufficiently larger than the diameter of the input
rod 284. Thus, an appropriate clearance is formed between the periphery of
the input rod 284 and the through hole 282.
An end of the input rod 284 located on the forward side of the vehicle is
connected to the pressing force transmitting member 270, and the other end
of the input rod 284 located on the rearward side of the vehicle is
connected to the brake pedal shown in FIG. 6. An end of a spring 286 is
engaged with the input rod 284. The other end of the spring 286 is engaged
with the cylindrical part 278 of the control valve 276. The spring 286
urges the input rod 284 and the pressing force transmitting member 270
toward the brake pedal 202 relative to the cylindrical part 278, that is,
the booster 262. When a brake pressing force is not input to the input rod
284, the input rod 284 and the pressing force transmitting member 270 are
held at a reference point shown in FIG. 1 by the above-mentioned urging
force generated by the spring 286.
An end of a spring 288 is also engaged with the input rod 284. The other
end of the spring 288 contacts the flat part 280 of the control valve 276.
An urging force of the spring 288 serves as a force to urge the flat part
280 toward the air valve 272.
When the pressing force transmitting member 270 is held at the reference
position as shown in FIG. 7, no force against the urging force of the
spring 288 is exerted on the flat portion except for a reaction force
generated by the air valve 272. Accordingly, when the pressing force
transmitting member 270 is located at the reference point, the flat part
280 is maintained to be in contact with the air valve 272. The diameter of
the air valve 272 is set to be larger than the diameter of the through
hole 282 of the control valve 276. Accordingly, under such a condition, a
state in which the through hole 282 is closed by the air valve 272 is
established.
The booster piston is provided with an annular valve seat 290 at a position
opposite to the flat part 280 of the control valve 276. The valve seat 290
is formed so that a predetermined clearance is maintained between the
valve seat 290 and the flat part 280 when the input rod 284 and the
pressing force transmitting member 270 are located at the reference
position. If there is a clearance between the valve seat 290 and the flat
part 280, the above-mentioned negative pressure passage 266 communicates
with the internal space 264. Additionally, if the valve seat 290 contacts
the flat portion 280, the negative pressure passage 266 is disconnected
from the internal space 264.
Air filters 292 and 294 are provided in the internal space 264 of the
booster piston 262. The internal space 264 is open to an atmospheric space
via the filters 292 and 294. Accordingly, an atmospheric pressure is
always introduced around the through hole 282 of the control valve 276.
The booster piston 262 contacts a reaction disc 296 at an end surface
located on the forward side of the vehicle. The reaction disc 296 is a
disc-like member formed by an elastic material. The other surface of the
reaction disc 296 contacts an output rod 298. The output rod 298 is a
member which is connected to an input shaft of the master cylinder 206
shown in FIG. 6. When a brake pressing force is exerted on the brake pedal
202, a pressing force corresponding to the brake pressing force is
transmitted to the master cylinder via the output rod 298. On the other
hand, a reaction force corresponding to the master cylinder pressure PM/C
is input to the reaction disc 296.
The center of the reaction disc 296 is opposite to the pressing force
transmitting part 274 of the pressing force transmitting member 270. The
pressing force transmitting member 270 is formed so that a predetermined
clearance is formed between the pressing force transmitting part 274 and
the reaction disc 296 when the pressing force transmission member 270 is
located at the reference position with respect to the booster piston 262.
A description will now be given of an operation of the brake force control
apparatus according to the present embodiment. In the present embodiment,
similar to the ECU 10 of the above-mentioned first embodiment, the ECU 200
determines whether the brake assist control should be started by
performing a routine shown in FIG. 4, and determines whether the brake
assist control should be continued by performing the routine shown in FIG.
5.
That is, the ECU 200 selects an appropriate condition from among the start
conditions (I) to (III) shown in FIG. 3 based on the elapsed time T after
the brake pedal 202 is pressed and the assumed vehicle speed VSO. Then,
the ECU 202 continues the normal control when the master cylinder pressure
PM/C detected by the hydraulic pressure sensor 212 and the rate of change
.DELTA.PM/C thereof do not satisfy the selected start condition, and, on
the other hand, starts the brake assist control when PM/C and .DELTA.PM/C
satisfy the selected start condition. Further, when a sufficiently strong
braking operation is performed after the brake assist control is started,
the ECU 200 cancels the execution of the brake assist control.
In the system according to the present embodiment, when the ECU 200
performs the normal control, both the negative pressure introducing valve
252 and the atmospheric pressure introducing valve 254 are maintained to
be in the OFF state. In this case, a negative pressure is introduced into
the negative pressure chamber 240 of the vacuum booster 204, and a
negative pressure is also introduced into the second pressure-changing
chamber 244. A description will now be given of an operation of the vacuum
booster 204 under such a condition.
When the brake pressing force FP is not applied to the brake pedal 202, the
input rod 284 and the pressing force transmitting member 270 are held at
the reference position (position shown in FIG. 7). In this case, a state
in which the air valve 272 is seated on the flat part 280 of the control
valve 276, and the flat part 280 is separated from the valve seat 290,
that is, a state in which the pressure changing passage 268 is
disconnected from the atmospheric space and communicates with the negative
pressure passage 266, is formed.
Under such a condition, the second pressure-changing chamber 244
communicates with the first pressure-changing chamber 242. Accordingly, a
pressure inside the first pressure-changing chamber becomes a negative
pressure similar to the pressure inside the second pressure changing
chamber 244 and the pressure inside the negative pressure chamber 240.
When the pressure inside the first pressure-changing chamber 242 is equal
to the pressure inside the second pressure-changing chamber 244, no force
caused by the negative pressures is exerted on the diaphragm 236.
Therefore, when the brake pressing force FP is not input, a pressing force
is not transmitted from the output rod 298 to the master cylinder 206.
When the brake pressing force FP is applied to the brake pedal 202, the
input rod 284 is moved relative to the booster piston 262 in the forward
direction of the vehicle, that is, in the rightward direction in FIG. 7.
When a relative displacement of the input rod 284 reaches a predetermined
length, an end surface of the pressing force transmitting part 274
contacts the reaction disc 296, and the flat part 280 of the control valve
276 seats on the valve seat 290 of the booster piston 262 so that the
negative pressure passage 266 is disconnected from the pressure changing
passage 268.
If the input rod 284 is further pressed in the direction toward the
reaction disc 296, the input rod 284 and the pressing force transmitting
member 270 continues to move while elastically deforming the center part
of the reaction disc 296, that is, a part of the reaction disc 296
(hereinafter, simply referred to as a center part) which contacts the
pressing force transmitting part 274. If the relative displacement of the
pressing force transmitting member 270 is increased as mentioned above, a
reaction force corresponding to an elastic deformation, that is, an
elastic force corresponding to the brake pressing force FP, is transmitted
to the input rod 284.
Additionally, after a state in which the flat part 280 is seated on the
valve seat 290 is established as mentioned above, the displacement of the
flat part 280 relative to the booster piston 262 is restricted. Thus, if
the input rod 284 is further pressed in the direction toward the reaction
disc 296 after such a condition is established, the air valve 272 is
separated from the flat part 280 of the control valve 276, and the
pressure changing passage 268 communicates with the through hole 282.
If such a state is established, an atmospheric air is introduced into the
first pressure-changing chamber 242 via the through hole 282 and the
pressure changing passage 268. As a result, the pressure inside the first
pressure-changing chamber 242 becomes higher than the pressure inside the
second pressure-changing chamber 244 and the negative pressure chamber
240. As mentioned above, if a pressure difference .alpha.PB is generated
between the first pressure changing chamber 242 and each of the second
pressure changing chamber 244 and the negative pressure chamber 240, a
pressing force FA (hereinafter, referred to as brake assist force FA)
which urges the diaphragm 236 in a direction toward the front of the
vehicle is exerted on the diaphragm 236.
It should be noted that the brake assist force FA can be approximately
represented by the following equation by using an effective
cross-sectional area SB of the negative pressure chamber 240 and an
effective crosssectional area SC of the second pressure changing chamber
244.
FA=(SB+SC).multidot..DELTA.PB (2)
The thus-generated brake assist force FA is transmitted from the diaphragm
236 to the booster piston 262, and further transmitted to a periphery of
the reaction disc 296, that is, a part of the reaction disc (hereinafter,
simply referred to as a peripheral part) which contacts the booster piston
262.
When the brake assist force FA is input from the booster piston to the
peripheral part of the reaction disc 296, an elastic deformation is
generated in the peripheral part of the reaction disc 296. This elastic
deformation increases as a pressure difference .alpha.P between opposite
sides of the diaphragm 236 increases, that is, as the introduction of air
into the first pressure changing chamber 242 is continued.
In the process in which an amount of elastic deformation in the peripheral
part of the reaction disc 296 is increased as mentioned above, the booster
piston is moved relative to a reaction force transmitting part 28 in the
direction toward the front of the vehicle. Then, if the amount of elastic
deformation of the peripheral part of the reaction disc 296 reaches a
value almost equal to the amount of elastic deformation of the center part
of the reaction disc 296, the flat part 280 of the control valve 276
contacts the air valve 272, and the introduction of atmospheric air to the
first pressure changing chamber 242 is stopped.
As a result, the pressure difference .alpha.P generated between opposite
sides of the diaphragm 236 is adjusted to a value corresponding to the
brake force FP input to the input rod 284. Additionally, the brake assist
force FA=(SB+SC) .alpha.PB becomes a value corresponding to the brake
pressing force FP. At this time, a resultant force of the brake assist
force FA and the brake pressing force FP is transmitted to the master
cylinder 206.
When the resultant force of the brake assist force FA and the brake
pressing force FP is transmitted to the master cylinder 206, the master
cylinder 206 generates a master cylinder pressure PM/C having a
predetermined power ratio with respect to the brake pressing force FP.
The ECU 200 turns off the SH 216 and SR 220 so as to set the hydraulic
circuit connected to the master cylinder 206 to a normal state. When the
hydraulic circuit is set to the normal state, the master cylinder pressure
PM/C is introduced into the wheel cylinder 218 as it is. Accordingly, the
brake force generated in the wheel cylinder 218 is adjusted to a level
corresponding to the brake pressing force FP.
If a slip rate S of a wheel exceeds a predetermined value after the braking
operation is started, the ECU 200 starts the ABS control similar to the
ECU 10 of the above-mentioned first embodiment. The ABS control is
performed when the brake pedal 202 is pressed, that is, when the master
cylinder pressure PM/C is appropriately increased.
Under the condition in which the master cylinder pressure PM/C is
appropriately increased, the SH 216 is set to the open state and the SR
220 is set to the closed state, and, thereby, the wheel cylinder pressure
PW/C is increased with the master cylinder pressure PM/C as an upper limit
value. Hereinafter, this state is referred to as a pressure-increasing
mode 1. Additionally, the wheel cylinder pressure PW/C is maintained
without being increased or decreased by the SH 216 being set to the closed
state and the SR 220 being set to the closed state. Hereinafter, this
state is referred to as a holding mode 2. Further, the wheel cylinder
pressure PW/C is decreased by the SH 216 being set to the closed state and
the SR 220 being set to the open state. Hereinafter, this state is
referred to as a pressure decreasing mode 3. The ECU 200 achieves, if
necessary, the above-mentioned pressure increasing mode 1, holding mode 2
and pressure decreasing mode 3 so that a slip rate S of the wheel becomes
an appropriate value.
When a depression of the brake pedal 202 is released by the driver during
execution of the ABS control, the wheel cylinder pressure PW/C must be
immediately decreased. In the system according to the present embodiment,
the check valve 222 is provided in the hydraulic circuit corresponding to
the wheel cylinder 218. The check valve 222 permits a fluid flow only in
the direction from the wheel cylinder 218 to the master cylinder 206.
Thus, according to the system of the present embodiment, the wheel
cylinder pressure PW/C of the wheel cylinder 222 can be inmediately
decreased after the depression of the brake pedal 202 is released.
In the system according to the present embodiment, when the ABS control is
performed, the wheel cylinder pressure PW/C is increased by the master
cylinder 206 as a fluid pressure source. Additionally, the wheel cylinder
pressure PW/C is decreased by having the brake fluid in the wheel cylinder
flow to the reservoir 224. Accordingly, if the pressure-increasing mode
and the pressure-decreasing mode are repeatedly performed, the brake fluid
in the master cylinder 206 gradually flows to the reservoir 224.
However, in the system according to the present embodiment, the brake fluid
in the reservoir 224 is delivered to the master cylinder 206 by the pump
226. Thus, if the ABS control is continued for a long time, a so-called
bottoming of the master cylinder does not occur.
A description will now be given of an operation achieved by the ECU 200
performing the brake assist control. As mentioned above, when the master
cylinder pressure PM/C and the rate of change PM/C thereof satisfy the
predetermined start condition, the ECU 200 starts the brake assist
control. The brake assist control is achieved by turning on both the
negative pressure introducing valve 252 and the atmospheric pressure
introducing valve 254, that is, by closing the negative pressure
introducing valve 252 and opening the atmospheric pressure introducing
valve 254.
The ECU 200 maintains both the negative pressure introducing valve 252 and
the atmospheric pressure introducing valve 254 to be set to the OFF state
until the ECU 200 determines that the start condition of the brake assist
control is established after the brake pedal 202 is pressed. Then, if it
is determined that the start condition is established, both the negative
pressure introducing valve 252 and the atmospheric pressure introducing
valve 254 are set to the ON state.
Until both the negative pressure-introducing valve 252 and the atmospheric
pressure introducing valve 254 are set to the ON state, the input rod 284
moves prior to the booster piston 262. As a result, the control valve 280
sits on the valve seat 290 and the air valve 272 separates from the
control valve 276. Thereby, atmospheric air is introduced into the first
pressure changing chamber 242, and the brake assist force
FA=(SB+SC).multidot..DELTA.PB is generated.
Under such a condition, if the negative pressure introducing valve 252 and
the atmospheric pressure introducing valve 254 are set to the ON state, a
pressure inside the first pressure changing chamber 242 and the second
pressure changing chamber 244 is rapidly increased to an atmospheric
pressure. As a result, a pressure difference .alpha.PAIR is generated
between the negative pressure chamber 240 and the first pressure changing
chamber 242. In this case, a brake assist force FA represented by the
following equation is exerted on the diaphragm 236.
FA=SB.multidot..DELTA.PAIR (3)
The brake assist force FA is transmitted from the diaphragm 236 to the
booster piston 262, and further transmitted to the peripheral part of the
reaction disc 296. Additionally, the brake pressing force FP which is
exerted on the brake pedal 202 is also transmitted to the reaction disc
296. Accordingly, thereafter, a resultant force of the brake assist force
FA and the brake pressing force FP is transmitted to the master cylinder
206.
In the system according to the present embodiment, similar to the
above-mentioned first embodiment, the brake assist control is started when
the brake pressing force FP is not sufficiently increased, that is, under
a condition in which a large brake assist force FA has not been obtained.
Accordingly, the brake assist force FA exerted on the booster piston 262
shows a sharp increase before or after the brake assist control is
started.
If the sharp change occurs in the brake assist force FA as mentioned above,
the booster piston 262 is rapidly and relatively moved toward the front of
the vehicle immediately after the brake assist control is started. Then,
when such a sharp change is generated in the booster piston 262, a
phenomenon occurs in which the control valve 276, which was seated on the
valve seat 290 before the brake assist control was started is separated
from the valve seat 290 when the control is started.
When the control valve 276 is separated from the valve seat 290, the second
pressure changing chamber 244 communicates with the first pressure
changing chamber 242. Accordingly, if a negative pressure is stored in the
second pressure changing chamber 244, the negative pressure is provided
from the second pressure changing chamber 244 to the first pressure
changing chamber 242 after the brake assist control is started. As a
result, there is a problem in that the brake assist force FA cannot be
raised imediately.
However, in the vacuum booster 204 of the present embodiment, atmospheric
air is introduced into the second pressure-changing chamber 244 at the
same time the brake assist control is started. Thus, according to the
system of the present embodiment, if the phenomenon in which the control
valve 276 is separated from the valve seat 290 after the brake assist
control is started occurs, the brake assist force FA can be raised
immediately.
The ECU 200 sets the hydraulic circuit connected to the master cylinder 216
to a normal state after the execution condition of the brake assist
control is established and until the execution condition of the ABS
control is established. In this case, the master cylinder pressure PM/C is
introduced to the wheel cylinder 218 without change. Accordingly, the
wheel cylinder pressure PW/C is rapidly increased from a pressure
corresponding to "(SB+SC).multidot..DELTA.PB+FP" to a pressure
corresponding to "SB.multidot..DELTA.PAIR+FP" when the brake assist
control is started.
As mentioned above, according to the system of the present embodiment, when
an emergency braking operation is performed, the wheel cylinder pressure
PW/C can be rapidly increased to a value sufficiently larger than the
brake pressing force FP. Thus, according to the system of the present
embodiment, a large brake force can be generated immediately after
establishmient of a condition in which an emergency braking is required,
even if the driver is a beginner-grade driver.
After the wheel cylinder pressure PW/C is rapidly increased as mentioned
above, the slip rate S of the wheel is rapidly increased, and finally the
execution condition of the ABS control is established. After the execution
condition of the ABS control is established, the ECU 200 achieves, if
necessary, the above-mentioned pressure increasing mode 1, holding mode 2
and pressure decreasing mode 3 so that a slip rate S of the wheel becomes
an appropriate value.
In the system according to the present embodiment, in a period during which
the brake pressing force FP is applied to the brake pedal 202 after the
brake assist control is started, the master cylinder pressure PM/C is
maintained to be a pressure corresponding to "SB.multidot..DELTA.PAIR+FP".
On the other hand, if a depression of the brake pedal 202 is released
after the brake assist control is started, the master cylinder pressure
PM/C is decreased to a pressure corresponding to
"SB.multidot..DELTA.PAIR".
Accordingly, by monitoring the output signal of the master cylinder
pressure PM/C detected by the hydraulic pressure sensor 212, the ECU 200
can determine whether or not the depression of the brake pedal 202 is
released. Upon detection of the release of the depression of the brake
pedal 202, the ECU 200 stops supply of the drive signals to the negative
pressure introducing valve 252 and the atmospheric pressure introducing
valve 254, and terminates the brake assist control.
It should be noted that the brake force control apparatus according to the
above-mentioned second embodiment is similar to the brake force control
apparatus according to the above-mentioned first embodiment in the
following points providing superior effects:
1+L when the driver operates the brake pedal 202 at a high speed without
intending to rapidly decelerate the vehicle, an erroneous start of the
brake assist control can be prevented;
2+L the vehicle can be rapidly decelerated by performing the brake assist
control when the driver is a beginner-grade driver, and the vehicle can be
rapidly decelerated by prohibiting execution of the brake assist control
when the driver is a high-grade driver;
3+L superior braking ability and superior riding quality can be
incompatible with each other in the entire vehicle speed range;
4+L the brake assist control can be appropriately started along with the
driver's intention both in a case in which the brake pedal 202 is pressed
with an intention to perform an emergency braking from the beginhing and
in a case in which an emergency braking is intended after the brake pedal
202 is pressed; and
5+L execution of the brake assist control already started can be
appropriately canceled when an emergency braking operation started by a
relatively gentle operation is, thereafter, changed to a rapid operation.
It should be noted that, in the above-mentioned second embodiment, although
the master cylinder pressure PM/C is used as the basic parameter for
discriminating between a normal braking operation and an emergency braking
operation, the basic parameter is not limited to this, and, similar to the
first embodiment, the brake pressing force FP the pedal stroke L, the
vehicle deceleration G, the assumed vehicle speed VSO or the vehicle speed
VW** may be used as the basic parameter.
A description will now be given, with respect to FIG. 8, of a third
embodiment of the present invention. FIG. 8 shows a system structure
diagram of a brake force control apparatus according to the present
embodiment. It should be noted that, in FIG. 8, only a part of the brake
force control apparatus corresponding to a single wheel is shown.
Additionally, in FIG. 8, parts that are the same as the parts shown in
FIG. 6 are given the same reference numerals, and descriptions thereof
will be omitted.
The brake force control apparatus shown in FIG. 8 is controlled by an ECU
300. In the brake force control apparatus according to the present
embodiment, a vacuum booster 302 is connected to the brake pedal 202. The
vacuum booster 302 is an apparatus which assists a brake pressing force by
using an intake negative pressure of an internal combustion engine as a
power source. The vacuum booster 302 used in the present embodiment is
different from the vacuum booster in the second embodiment, and is a
general apparatus which always assists the brake pressing force FP with a
constant power ratio.
In the system according to the present embodiment, the reservoir 224
communicates with a fluid pressure passage 304 which communicates with the
reservoir tank 208. The fluid pressure passage 304 is provided with a
check valve 306 and a switching solenoid 308 (hereinafter, referred to as
SCH 308). The check valve 306 is a one-way valve which permits a fluid
flow only in a direction from the reservoir tank 208 to the reservoir 224.
Additionally, the SCH 308 is a two-position solenoid valve which maintains
a closed state in a normal state (OFF state). The SCH 308 is opened by a
drive signal being supplied from the ECU 300.
The fluid pressure passage 210 is provided with a fluid pressure cutting
solenoid 214 (hereinafter, referred to as SC 214). The SC 214 is
two-position solenoid valve which opens and closes the fluid pressure
passage 210, and maintains an open state in a normal state (OFF state).
The SC 214 is set to an ON state (closed state) by a drive signal being
supplied from the ECU 300.
A description will now be given of an operation of the brake force control
apparatus according the present embodiment. Similar to the ECU 10 of the
above-mentioned first embodiment and the ECU 200 of the above-mentioned
second embodiment, the ECU 300 of the present embodiment determines
whether the brake assist control should be started by performing a routine
shown in FIG. 4, and determines whether the brake assist control should be
continued by performing the routine shown in FIG. 5.
That is, the ECU 300 selects an appropriate condition from among the start
conditions (I) to (III) shown in FIG. 3 based on the elapsed time T after
the brake pedal 202 is pressed and the assumed vehicle speed VSO. Then,
the ECU 300 continues the normal control when the master cylinder pressure
PM/C detected by the hydraulic pressure sensor 212 and the rate of change
.DELTA.PM/C thereof do not satisfy the selected start condition, and, on
the other hand, starts the brake assist control when PM/C and .DELTA.PM/C
satisfy the selected start condition. Further, when a sufficiently strong
braking operation is performed after the brake assist control is started,
the ECU 300 cancels the execution of the brake assist control.
In the system according to the present embodiment, when the ECU 300
performs the normal control, all of the SC 214, the SCH 308, the SH 216
ant the SR 220 are maintained to be in the OFF state, and the pump 226 is
maintained to be stopped. In such a condition, only the master cylinder
206 can serve as a fluid pressure source, and the master cylinder pressure
PM/C generated in the master cylinder 206 is supplied to the wheel
cylinder 218. Accordingly, in this case, the wheel cylinder pressure PW/C
of the wheel cylinder 218 is adjusted to a fluid pressure having a
predetermined power ratio.
If the slip rate S of the wheel exceeds a predetermined value, similar to
the ECU 200 of the above-mentioned second embodiment, the ECU 300 starts
the ABS control. The ABS control can be achieved by operating the pump 226
and by realizing the above-mentioned pressure increasing mode 1, holding
mode 2 and pressure decreasing mode 3 so that the slip rate S of the wheel
becomes an appropriate value.
When the master cylinder pressure PM/C and the rate of change .DELTA.PM/C
thereof satisfy a predetermined start condition, the ECU 300 starts the
brake assist control. In the system according to the present embodiment,
the brake assist control is achieved by turning on both the SC 214 and the
SCH 308, that is, by closing the SC 214 and opening the SCH 308, and
operating the pump 226.
Under such a condition, the master cylinder 206 and the wheel cylinder 218
are disconnected from each other. On the other hand, the pump 226 delivers
the brake fluid supplied from the reservoir tank 208 via the fluid
pressure passage 304 toward the wheel cylinder 218. Thus, the wheel
cylinder pressure PW/C of the wheel cylinder 218 is increased by the pump
226 as a fluid pressure source.
The pump 226 is capable of rapidly increasing the wheel cylinder pressure
PW/C immediately after the brake assist control is started. Accordingly,
when execution of an emergency braking operation is detected by the ECU
300, the wheel cylinder pressure PW/C of the wheel cylinder 218 is rapidly
increased irrespective of whether the brake pressing force FP is large or
small.
As mentioned above, according to the system of the present embodiment, when
an emergency braking operation is performed, the wheel cylinder pressure
PW/C of the wheel cylinder 218 can be rapidly increase to a sufficiently
large value irrespective of the brake pressing force FP. Thus, according
to the system of the present embodiment, a large brake force can be
generated immediately after establishment of a condition in which an
emergency braking is required is established even if the driver is a
beginner-grade driver.
After the wheel cylinder pressure PW/C is rapidly increased as mentioned
above, the slip rate S of the wheel is rapidly increased, and, finally,
the execution condition of the ABS control is established. After the
execution condition of the ABS control is established, the ECU 300
achieves, if necessary, the above-mentioned pressure increasing mode 1,
holding mode 2 and pressure-decreasing mode 3 so that the slip rate S of
the wheel becomes an appropriate value.
In the system according to the present embodiment, in a period during which
the brake assist control is performed, the SC 214 is maintained in the ON
state. If the SC 214 is in the ON state, the fluid pressure chamber of the
master cylinder 206 and a part of the upstream side of the SC 214 of the
fluid pressure passage 210 become substantially a closed space.
Under such a condition, the master cyl nder pressure PM/C is a value
corresponding to the brake pressing force FP. Accordingly, by monitoring
the output signal of the master cylinder pressure PM/C detected by the
hydraulic pressure sensor 212, the ECU 300 can easily determine whether or
not the depression of the brake pedal 202 is released. Upon detection of
the release of the depression of the brake pedal 202, the ECU 300 stops
supply of the drive signals to the SC 214 and the SCH 308, and terminates
the brake assist control.
It should be noted that the brake force control apparatus according to the
above-mentioned third embodiment is similar to the brake force control
apparatus according to the above-mentioned first embodiment in the
following points providing superior effects that:
1+L when the driver operates the brake pedal 202 at a high speed without
intending to rapidly decelerate the vehicle, an erroneous start of the
brake assist control can be prevented;
2+L the vehicle can be rapidly decelerated by performing the brake assist
control when the driver is a beginner-grade driver, and the vehicle can be
rapidly decelerated by prohibiting execution of the brake assist control
when the driver is a high-grade driver;
3+L superior braking ability and superior riding quality can be compatible
with each other in the entire vehicle speed range.
4+L the brake assist control can be appropriately started along with the
driver's intention both in a case in which the brake pedal 202 is pressed
with an intention to perform an emergency braking from the beginning and
in a case in which an emergency braking is intended after the brake pedal
202 is pressed; and
5+L execution of the brake assist control already started can be
appropriately canceled when an emergency braking operation started by a
relatively gentle operation is, thereafter, changed to a rapid operation.
It should be noted that, in the above-mentioned third embodiment, although
the master cylinder pressure PM/C is used as the basic parameter for
discriminating between a normal braking operation and an emergency braking
operation, the basic parameter is not limited to this, and, similar to the
first embodiment, the brake pressing force FP, the pedal stroke L, the
vehicle deceleration G, the assumed vehicle speed VSO or the vehicle speed
VW** may be used as the basic parameter.
It should be noted that, in the above-mentioned first to third embodiments,
although the brake assist is always performed when a braking operation
satisfying the execution condition is performed, a structure may be used
in which the execution of the brake assist control can be prohibited by a
manual operation of the driver by providing an on/off switch regarding the
brake assist control in the vehicle compartment.
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